Method and apparatus for heating the bed material in a PFBC plant

ABSTRACT

A method of heating of bed material in a PFBC plant including a combustor and wherein bed level adjustment due to a load change is accomplished by injecting bed material into or discharging it from the combustor through feeding devices, the method including the steps of arranging two storage vessels, each adapted for injecting the bed material therefrom into the combustor and for storing bed material discharged from the combustor, and heating and maintaining the bed material in the storage vessels at the temperature corresponding to the bed temperature while maintaining the bed level of the combustor constant, by discharging bed material with the temperature of the bed from the combustor via the feeding device adapted for discharge of bed material, into one of the storage vessels with a predetermined feed speed and at the same time and at the same feed speed injecting bed material from the second storage vessel into the combustor via the feeding device adapted for injection of bed material.

TECHNICAL FIELD

The present invention relates to a method and to a device whichconstitute an integral part of a PFBC plant. Adjustment of the bed levelin such a plant in connection with load changes is performed, amongother things, with the aid of bed material which, from a storage vessel,is supplied to or discharged from the combustor of the plant. In ordernot to disturb the temperature control of the bed too much in connectionwith the bed level adjustment, the temperature of the supplied bedmaterial must not differ too much from the temperature of the bed. Thepurpose of the invention is to heat the stored bed material by a specialmethod such that the disturbances in the temperature control of the bedare as small as possible.

BACKGROUND OF THE INVENTION

EP 0124842 describes a "Power plant with a combustor with a fluidizedbed". The combustor burns the fuel supplied to the fluidized bed. Theplant is provided with means for adjusting the bed level in dependenceon the operating conditions by the transfer of bed material from thecombustor to a storage vessel and vice versa, wherein the storage vesselfor bed material is connected to the combustor by means of discharge andreinjection conduits and the storage vessel is connected, via a conduitprovided with a valve, to a space with a pressure lower than that in thecombustor, and wherein the reinjection conduit is connected, via aconduit provided with a valve, to a pressure gas source, the pressure ofwhich is higher than the pressure in the combustor. Both the combustorand storage vessel of the plant are enclosed within a pressure vessel.

Otherwise, the prior art regarding means for handling bed material inplants with a combustor with a fluidized bed is described in detail in areport ANL/CEN/FE-81-3 from Argonne National Laboratory, Argonne, Ill.,entitled "Discharge and Handling of Solids from Pressurized FluidizedBed Combustors" by John E. Hanway Jr. and W. F. Podolski.

A PFBC plant comprises a large number of more or less interdependentcontrol systems with various forms of external limitations. An importantcontrol system in this connection is the control which is designed tokeep the bed at a constant temperature of about 850 degrees centrigradeindependently of the output of power from the plant.

There is a relatively clear-cut relationship between the available poweroutput from the plant and the bed level. To be able to maintain aconstant power output, there must be a continuous supply of the coalmixture which is burnt. The ashes which are then generated tend toincrease the amount of bed material in the bed, whereby the bed leveltends to rise. The fact that this does not take place is due to the factthat a bed level control with a set value corresponding to the desiredand current bed level ensures that an amount of bed material isdischarged which is equal to the production of new bed material duringcombustion.

When a load change occurs, the bed level needs to be changed to beadapted to the new load condition. Since the production of new bedmaterial in spite of all is relatively modest, the rate of increasewhich would be obtained in the bed if the discharge of bed material werestopped is normally far from sufficient to provide the desired speed ofthe bed level change in case of an increased load requirement. It istrue that, in case of a reduction of load, the speed of the bed materialdischarge could be increased; however, such a method alone could notgenerally take care of load changes. As is clear from the above, in themethod disclosed in the EP patent the bed level is adjusted upon a loadchange by injecting more bed material from the storage vessel in case ofincreased power requirement in order thus to raise the bed level, andthat bed material is sucked out from the bed to be able to lower the bedlevel.

The above is, per se, a simple, relatively fast and safe method foradjusting the bed level upon a load change. However, this method imposesvery high demands on the temperature control of the bed, in fact so highdemands that it is a factor limiting the rate at which load changes cantake place. The reason for this is that, although the storage vessel ofthe bed material is located inside the pressure vessel, the bed materialstored in the storage vessel has a considerably lower temperature thanthe bed. To reduce the disturbing influence on the temperature controlwhich is caused by this injection and discharge of bed material, itwould be desirable if the bed material in the storage vessel had atemperature which largely corresponds to the bed temperature. A heatingof the bed material may, of course, be performed in a plurality ofdifferent ways. The present invention describes an alternative way ofheating the bed material in the storage vessel while utilizing the heatstorage constituted by the bed itself.

SUMMARY OF THE INVENTION

A device for heating bed material according to the present inventioncomprises two parallel working, identical storage vessels. Upon a loadincrease, bed material is injected from the two vessels into the bottomof the bed to rapidly raise the bed level. In a corresponding way, bedmaterial is discharged from the bottom of the bed and back to the twovessels upon a load decrease. The collected bed material located in thetwo vessels corresponds to half of the total volume of the vessels andin a conceived initial position both vessels are half-filled. Thevessels are provided with minimum and maximum limitations with respectto the volume of the bed material.

Heating of bed material and maintenance of the temperature of the bedmaterial at a temperature largely corresponding to the bed temperaturetake place under the operating conditions when the load or the poweroutput is practically constant. As is clear from the above, constantload means that the bed level is to be constant. By injecting bedmaterial into the bottom of the bed from one of the storage vessels,hereinafter in the description called the first storage vessel, and atthe same time discharging an equal amount of the bed material located inthe bottom of the bed, with the temperature of the bed, into the othervessel, the bed level will remain constant corresponding to the currentconstant power output. Warm bed material will then be supplied to thesecond storage vessel. When this vessel starts to be filled, or when thebed material in the first vessel starts to approach its lowerlimitation, the direction of the bed flow is changed such that bedmaterial from the second vessel is reinjected into the bed and an equalamount of bed material is discharged from the bed and supplied to thefirst vessel. The sequence of change is interrupted when the level ofthe bed material in the two storage vessels has become equally high.

Ensuring that the bed level in the combustor remains constant whileperforming simultaneous injection and discharge to and from the storagevessels requires a control system which ensures that injected anddischarged bed material is at all times equal. The invention thereforeincludes an interdependent control system between the injection anddischarge devices which is activated when the bed material in thestorage vessels needs to be heated, that is, when the temperature of thestored bed material drops below a predetermined value. The speed ofinjection of bed material from the first vessel to the bed is mainlycontrolled by a bias signal which, via a P-connected amplifier, suppliesthe necessary control signal to the injection device. This control isincluded in an external bed level control with a locked set value. Thislocked set value corresponds to the load prevailing at the beginning ofthe heating process. The actual value of the bed level control is equalto the bed level continuously measured during the heating process. Thedeviation between these two values is supplied both to the P-connectedamplifier, connected in opposition to the bias signal, and to aPI-connected amplifier, the output signal of which supplies thenecessary control signal to the discharge device from the bed, that is,the injection device for the second vessel.

When the heating sequence is activated, the set and actual values forthe bed level are equal and, consequently, the control deviation iszero. It is then the set value of the bias signal which, via theP-connected amplifier, supplies a control signal for the injection speedfor bed material from the first vessel and into the bed. The injectionresults in a tendency of the bed level to rise and gradually a controldeviation occurs between the locked set value and the actual value. Assoon as a control deviation arises, the discharge device will start independence on a signal of the PI-connected amplifier. At the same time,the control deviation will reduce the injection speed via theP-connected amplifier. In this way, after some time, injection anddischarge will operate with the same rate of feed.

The sequence during heating may either be started manually orautomatically when the temperature of the bed material in the first orsecond vessel has become too low, provided that the power output fromthe plant is constant.

If an order to change the bed-level because of a load change arrivesfrom a superordinate control, this load change may be either delayeduntil the heating process is completed, or the heating process may beinterrupted. The purpose of the limitations for the vessels as regardsthe maximum and minimum levels of bed material is to have, under allconditions, available bed material and available space in any of thevessels for injection and discharge of bed material to adapt to the newpower requirement.

The use of two storage vessels, as shown above, allows the bed materialwhich upon a load change is supplied to the bed to have largely the sametemperature as the remaining part of the bed. This means that thetemperature control of the bed is disturbed to a considerably lesserdegree than when using the technique described, for example, in EP0124842. This is, of course, of great importance for the dimensioning ofthe temperature control and provides great advantages in relation to theprior art.

BRIEF DESCRIPTION OF THE DRAWINGS

The sole FIGURE shows a part of a PFBC plant which deals with injectionand discharge of bed material in connection with load changes.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The accompanying single FIGURE shows that part of a PFBC plant whichdeals with injection and discharge of bed material in connection withload changes. The bed inside a combustor 1 has a bed level equal to h.Upon a load change ΔL, the bed level is adjusted by injection ordischarge of bed material 2 from two storage vessels 3 and 4. When theload is increased, the bed material is to be injected into the lowerpart of the combustor from the two storage vessels and in acorresponding way, when the load is decreased, bed material is to bedischarge from the lower part of the combustor into the two storagevessels. This is handled by a ΔL control 5.

In a PFBC plant according to the Figure, injection and discharge of bedmaterial to and from the combustor takes place by means of screw feeddevices 6 and 7 which are each driven by a motor 8 and 9 and powermembers 10 and 11. Another method for injection and discharge comprisesintroducing valves in the feed conduits between the storage vessels andthe combustor and, in connection with injection after having openedthese valves, supplying an overpressure to the storage valves andproviding a sub-atmospheric pressure in the vessels in connection withdischarge. Both the ΔL control and the method for heating the bedmaterial according to the invention are independent of the manner inwhich injection and discharge of bed material are performed but assumethat there are two storage vessels for bed material. When changingbetween normal load control and heating control, the control signals tothe power members 10 and 11 are switched with the aid of the switches 12and 13 between the ΔL control and the heating control.

As will have been clear from the description, a load change of the plantrequires adjustment of the bed level. The adjustment is performed by theabove-mentioned injection or discharge of bed material to or from thecombustor. It is also clear from the above that it is important that thetemperature of the bed is not changed too much, which may very wellhappen when bed material is supplied to or discharged from thecombustor. Thus, it is highly desirable that the material which isinjected into the combustor has a temperature which is as close to thebed temperature as possible. The invention therefore comprises a methodwhich, with the aid of the two vessels and the heat storage constitutedby the bed, heats the bed material such that this material when beinginjected into the combustor has a temperature which corresponds well tothe temperature of the bed.

The heating process is adapted so as not to disturb a load equalizationprocess. Therefore, the heating only takes place when the load ispractically constant. It is thus important that a constant bed level bemaintained during the heating process. For this purpose an external bedlevel control is provided during the heating, and this control ensuresthat the bed level prevailing when the heating sequence starts remainspractically constant. The value of the bed level at the start of theheating is locked as a set value for the bed level control during theentire heating process. This value is compared in a summator 14 with thecurrent value of the bed level at all times during the heating.

When the heating process starts, the feed device for the storage vesselfrom where bed material is to be injected into the bed, that is, fromstorage vessel 3, the first storage vessel, in the operating state shownin the Figure, receives a control signal from a P-connected amplifierwith a bias input signal 16, whereby the injection into the bed isinitiated. This means that the current value of the bed level startsincreasing and a control deviation occurs in the form of an outputsignal from the summator 14. The control deviation signal is integratedinto the PI-connected amplifier 17, whereby also the discharge devicefrom the bed, that is, the injection device to the second storage vessel4, receives a control signal and the discharge from the bed starts. Thecontrol deviation signal is also supplied to the P-connected amplifierwith such a polarity that the speed of injection of bed material to thebed is reduced. This means that the injection speed and the dischargespeed of bed material to and from the bed approach one and the samevalue corresponding to the bias obtained from the setting device 16 andthat this value is reached after a short synchronization time.

When bed material during the heating is to be moved in a directionopposite to that described above, that is, from the second storagevessel 4 via the bed to the first storage vessel 3, the control signalis connected from the P-connected amplifier 15 to the power member 11for operation of the feed device associated with this second storagevessel in the opposite direction while at the same time the controlsignal is connected, via the PI-connected amplifier, to the power member10 for the first storage vessel.

We claim:
 1. A method of heating bed material in a PFBC plant having a combustor and in which bed level adjustment due to a load change is accomplished by injecting or withdrawing bed material into/from said combustor, said heating method including the steps of:1) heating and storing bed material in at least two storage vessels to maintain said bed material available for bed level adjustment at a temperature substantially corresponding to the bed material temperature; wherein said heating step includes:a) positioning feeding devices between each of said storage vessels and the combustor; b) feeding out bed material from said combustor through one of said feeding devices into one of said at least two storage vessels; c) feeding into said combustor equal amounts of bed material from another of said storage vessels through another of said feeding devices simultaneously with said step a); d) performing said feeding out and feeding into at the same speed; and 2) maintaining constant bed level of the combustor during said heating.
 2. A method for heating of bed material in a PFBC plant according to claim 1 wherein said bed material is only heated when the load of the PFBC plant is constant.
 3. A method for heating of bed material in a PFBC plant according to claim 1 wherein heating of bed material is interrupted when the bed level of the combustor needs to be adjusted because of a change of load.
 4. A method for heating of bed material in a PFBC plant according to claim 1 wherein the bed level of the combustor is maintained constant during the heating by a bed level control, the set value of the bed level being maintained constantly equal to the bed level when the heating starts and the actual value of which is a continuously measured value of the current bed level.
 5. A method for heating of bed material in a PFBC plant according to claim 1 wherein the speed of injection and discharge of bed material to and from the combustor is determined by an internal bias setting in the bed level control.
 6. A method of heating of bed material in a PFBC plant including a combustor and wherein bed level adjustment due to a load change is accomplished by injecting bed material into or discharging it from the combustor through feeding devices, said method including the steps of:arranging at least two storage vessels in communication with the combustor, each adapted for injecting bed material therefrom to the combustor and for storing therein bed material discharged from the combustor; heating and maintaining temperature of the bed material in the storage vessels at a temperature substantially corresponding to the bed temperature by:a) injecting bed material into the bed from one of the storage vessels while simultaneously discharging equal amounts of bed material from the combustor into the other storage vessel; b) reversing the direction of the bed material flow when the amount of bed material in one of the storage vessels reaches minimum or maximum volume limitation; and c) controlling simultaneous injection and discharge of the equal amount of bed material between the storage vessels and the combustor to ensure constant bed level in the combustor.
 7. An apparatus for heating bed material in a PFBC plant having a combustor and in which bed level adjustment due to a load change is accomplished by injecting or withdrawing bed material into/from said combustor, said heating apparatus including:a) at least two storage vessels for storing bed material available for said bed level adjustment at a temperature substantially corresponding to the bed material temperature; b) feeding devices between each of said storage vessels and the combustor designed for feeding bed material out and into each of said storage vessels; c) controlling means for controlling any one of said feeding devices to feed out bed material from said combustor at a predetermined speed and for simultaneously controlling another feeding device to feed an equal amount of the bed material into said combustor at the same predetermined speed, while maintaining constant bed material level in said combustor to thereby heat and maintain said bed material in said storage vessels at said temperature substantially corresponding to the bed material temperature. 